1. Field of the Invention
This invention relates to a heavy load pneumatic tire in which a tread portion has a rib pattern formed with ribs divided by main grooves which are continuous in a tire circumferential direction.
2. Description of the Related Art
Conventionally, as a tread pattern formed on a tread portion of a heavy load pneumatic tire, there is a known rib pattern formed with ribs divided by a main groove which is continuous in a tire circumferential direction. The heavy load pneumatic tire provided with the rib pattern is superior to a tire having a block pattern in wear resistance, but has a problem that only an edge of the rib is locally unevenly worn in the circumferential direction, i.e., a river wear is prone to be generated.
Concerning details of generation process of this river wear, it is considered that a fine step is first generated in a rib end by a lateral force caused during running of the tire, a drag is generated by a diameter difference at this step portion and this step spreads in a widthwise direction of the tire, and the irregular wear is developed. Thus, in order to suppress the river wear, it is extremely important to prevent the local wear of the rib edge.
As means for suppressing such a fine step of the rib edge, as shown in FIG. 8, generally, fine sipeings 11 are formed in a widthwise direction of an edge of a rib 5 to reduce the rigidity, thereby suppressing local wear (see Japanese Patent Application Laid-open No. 2004-1608, for example). One end of the sipeing 11 is opened toward a groove, and the other end is closed within the rib.
However, in order to effectively lower the rigidity by the sipeings, it is necessary to elongate the sipeing, there is an adverse effect that a crack is generated in the sipeing end by input from a road surface, and there is a problem that the irregular wear resistance is deteriorated, the appearance is deteriorated and the like.
On the other hand, there is a known tire in which a bottom surface of a main groove of a tire tread portion is provided with a projecting stripe, but this tire can not effectively prevent the river wear from generating due to the following reasons. For example, Japanese Patent Application Laid-open No. 2001-163014 discloses a tire in which a bottom surface of a main groove is provided with a projecting stripe whose height is 0.1 to 0.3 of the groove depth and whose width is 0.5 to 1.0 of the groove bottom width. Since the height of the projecting stripe is low and the width thereof is too wide, it is difficult to prevent the river wear generation while preventing the crack in the grove bottom or stone-biting. Japanese Patent Application Laid-open No. H9-66708 discloses a tire in which a bottom surface of a main groove is provided with a projecting stripe whose height is 0.15 to 0.25 of the groove depth and whose width is 0.20 to 0.35 of the groove bottom width. However, since the groove provided with the projecting stripe is a wide main groove having the groove width greater than the groove depth, the rigidity enhancing effect obtained by providing the projecting stripe becomes low, and it is difficult to effectively prevent the river wear generation.
In order to prevent the stone-biting toward the groove in the circumferential direction, there is a known tire in which a bottom surface of a groove of the tire tread portion in the circumferential direction is provided with a corrugated projecting stripe whose height is varied cyclically (see Japanese Patent Application Laid-open No. H6-239107, for example). However, since the groove bottom is provided with a simply corrugated projecting stripe, there is a problem that the river wear can not effectively be prevented.